Long before the Spanish arrived, Cauayan was home to the Gaddang people. These dark-skinned, possibly pygmy-related ancestors of the Aeta people first settled the area around 26,000 years ago.

Around 200 BC, another wave of settlers, the Indo-Malays, arrived in Luzon.  Among them were the Gaddangs, who practiced slash-and-burn farming and thrived along the Cagayan River and its tributaries. Their movement inland suggests they might have been some of the earliest Indo-Malay arrivals.

The Gaddangs became the founding population of Cauayan and neighboring towns. Their name itself reflects their darker complexion compared to other groups in the region, possibly due to the term "ga" meaning "heat" and "dang" meaning "burned."

 The Gaddangs were the original inhabitants of Cauayan, Isabela, before the Spanish colonization. The Spanish authorities forced the Gaddangs to live in the Spanish-held settlements of the plains and controlled the tobacco production through the Tobacco Monopoly and the establishment of tobacco haciendas. The Gaddangs continued to resist the Spanish colonization, with Don Juan Cauilian subduing the pagan Gaddangs of Siffu who continued to harass Christian communities in the towns of Cauayan, Carig, and Lappau in April 1754. The Spanish missionaries continued to evangelize the old Cauayan territories, with Fray Antonio del Campo going to Cauayan in June 1741, escorted with soldiers from the capital of Lal-lo. Together with Fray Luis Pedro de Sierra, they went up the mountains and convinced and brought down pagans as well as Christians apostates who had fled from Tagaran and Anaccuan. The Spanish authorities continued to control the tobacco production, with the Compania General de Tobacos de Filipinas or Tabacalera established in 1881 to continue the export of leaf tobacco and take over the cigar factories owned by the Spanish government. The tabacalera bought from the government all the tobacco factories in Manila, which wielded into a single factory called La Flor de Isabela, one of the largest of its kind in the world. The three tobacco haciendas were: Hacienda San Antonio in Ilagan, which was the largest and named in memory of Don Antonio Lopez y Lopez, 1st Marques de Comillas (1817-1883); Hacienda Santa Isabel also in Ilagan, which was named in memory of a daughter of Don Lope Gisbert; and Hacienda San Luis in Cauayan.

The Gaddang language is spoken in the Cagayan Valley, particularly in the provinces of Isabela and Nueva Vizcaya, in the Philippines. It is recognized as a distinct dialect with unique phonemes such as "F", "V", "Z", and "J" sounds, which are not commonly found in many neighboring Philippine languages. The Gaddang language is also noteworthy for the common use of doubled consonants, such as "Gad-dang" instead of "Ga-dang".

The Gaddang language is agglutinative, meaning that it features the addition of affixes to roots to convey meaning. This is in contrast to inflectional languages, which use changes in the form of words to convey meaning. The Gaddang language is characterized by a dearth of positional/directional adpositional adjunct words, and temporal references are usually accomplished using context surrounding these agglutinated nouns or verbs.

The Gaddang language is identified in Ethnologue and Glottolog, and is incorporated into the Cagayan language group in the system of linguistic ethnologist Lawrence Reid. The Dominican fathers assigned to Nueva Viscaya parishes produced a vocabulary in 1850, and in 1965, Estrella de Lara Calimag interviewed elders in the US and the Philippines to produce a word-list of more than 3,200 Gaddang words included in her dissertation at Columbia. The Austronesian Basic Vocabulary Database also lists translations of more than two hundred English terms on its Gaddang page.

However, the use of Gaddang as a primary language has been declining during the last seventy years. The adoption of the 1973 Constitution and its 1976 amendments discouraged the use of English in the schools of Isabela and Nueva Vizcaya, and the 1987 Constitution further marginalized local languages. Television and official communications have almost entirely used the national Filipino language for nearly a generation, contributing to the decline of the Gaddang language.